Guillermo Rein g.rein@ed.ac.uk School of Engineering, University of Edinburgh
El Mundo

1. LARGE-SCALE BIOCHAR SEQUESTRATION

5. RISK MITIGATING TECHNOLOGIES
These facilities should be self-sufficient and passive (no input of energy required). The addition of active detection and suppression system that do not require energy input is possible as well. The methods and technologies available for designing these facilities would drawn and combined concepts from: Stockpile size: As the size of the pile is made smaller, heat losses increase and the risk of self-heating is reduced. The maximum safe stockpile size is given by the ambient temperature (Section 6). Compartmentation: Storage of biochar in adjacent compartments separated by noncombustible barriers stopping the spread of a fire, imposing the maximum safe stockpile size and allowing for sealing. Ventilation: Design features enhancing natural ventilation and cooling Sealing: Storage in sealed compartments with low O2 (or inert) atmospheres. It is known that smouldering fires cannot propagate at [O2]<16% [4]. Wetting: Material with large water contents (>125% dry base) do not ignite [5]. Inertation: Reducing reactivity by mixing biochar with inert material (see Section 6).

Fire Safety Engineering

Biochar is a breakthrough technology allowing for efficient and sustainable sequestration of atmospheric carbon in the solid phase. In comparison to gasphase sequestration, biochar offers the additional advantages of well distributed production sites, plus cheaper and more stable transport and storage. Facilitates can be designed to store large stockpiles (~106 m3) on surface, marine or mining sites. This would represent the geo-engineering application of a negative-carbon technology with a low level of geointervention.

2. ACCIDENTAL RELEASE of CARBON
The major menace to the stability of biochar in the very long term (in the order of millennia) is fire. Biochar is a very flammable material. Moreover, as all carbon-rich solids, is known to be prone to self-heating (see Sections 3 and 5). If unchecked, it results in spontaneous ignition and can develop into a smouldering mega-fire [1], which would be difficult or impossible to suppress (Figure 2). This would lead to the accidental release to the atmosphere of the sequestrated carbon. It can be seen as the equivalent problem of leakage in carbon gas-phase sequestration.

6. EXAMPLE of INERTATION
The concept of inertation, to reduce reactivity by mixing biochar with inert material, has been proven for the first time in a series of experiments lead by the author. Cubic gauze baskets of various sizes were filled with particles of charcoal provided by UKBRC (as a proxy to biochar). Different quantities of clean quartz sand were mixed with the char (from 1 to 3 parts of sand). The reactivity of the mixture was measured for different basket sizes and oven temperatures. Kamenetski’s theory [3] allows then to obtain the maximum safe stockpile sizes for different temperatures (Figure 4). Inertation allows for 200 to 600% larger stockpile sizes, even for hot countries.
100 km After:
Reacted sample to ash

3. SELF-HEATING
Self-heating [2] refers to the tendency of certain reactive solids in oxidative atmospheres to spontaneous exothermic reactions at low or ambient temperatures. This is a well known problem for industries managing carbon-rich materials [3]. Initially, small amounts of heat are released and accumulate during longer times when heat losses are low (eg, large stockpiles, high ambient temperatures). This results in a sustained increase of temperature without an external heat source. Above a certain temperature (point 2 in Figure 3), the process self-accelerates and leads to thermal run away. Semenov’s Ignition Theory describes the process as controlled by heat generation and heat losses (Equation 1).
& ql
& q
2

Before:
Original char sample

BRE Centre

∂T = ρc 123 4 ∂t 4 Rate Change

A hT (T − T0 ) V 4 3 Self − heating 14 244 generation Heat losses

Hω ∆2 & − 13

National Geographic 2008/ AP Photo/MODIS

Heat

Figure 4– Maximum safe stockpile sizes for the range of ambient temperatures found in hot to mild climates

7. GEO-ENGINEERING
The Royal Society defined in 2009 Geo-Engineering as:

4. DESIGN of FACILITIES
In order to minimize and avoid the risk of an accidental release associated with biochar storage, a new type of large facilities for stable and very-longterm storage should be built. Facilitates can be designed to store on surface, marine or mining sites. The methods for designing these facilities would use technological concepts borrowed from infrastructure fire protection.
http://hdl.handle.net/1842/2678

“deliberate large-scale intervention in the Earth’s climate system, in order to moderate global warming”
The author thinks that designing and building facilities for very longterm sequestration of solid carbon would represent a engineering task at the Earth-scale and thus a Geo-Engineering topic, but with a lower level of geo-intervention compared to other proposal put forward.